I have a TWR-K64F120M board. I can use the OpenSDA connection with the Segger software and everything works fine. When I try to use the on board JTAG connector (J5) with my J-Link probe it fails complaining about the target voltage being too low. I'm powering the board using the OpenSDA mini-USB. Both the OpenSDA and the J-Link probe show up in the device list when I start the J-Link GDB server.
As a sanity check I tried this same set up with a TWR-K21F120M board and it also works fine. I don't know if it means anything but the board was shipped with a plastic clip over the JTAG connector.